I posted about this in my Intro thread in regards to my history with the Pokemon fandom in the early days, and well as my reason on why I left the fandom for some tome (see here: serebiiforums.com/showthread.php?653120-Wanting-to-get-back-into-the-Pokemon-fandom-(particularly-the-main-series-games) ) but basically, I am one of those that left the franchise about 11+ years ago and ever since Pokemon Go came out and seeing Pokemon Sun and Moon be advertised, I've kind of been feeling the Pokemon bug itch again, so much so that I'm willing to invest in a 3DS and Pokemon X/Y and ORAS and then Sun/Moon despite being in a different part of my life than I was 11+ years ago.
I say all this because I feel that I can attest in stating something in regards to Pokemon's popularity.
One thing that should be set straight is that the term "popular" is a relative term. Now that that's out of the way: in terms of popularity in mainstream society, no, it has not been at the level that it was when it was first introduced in the US back in the late 1990's/early 2000's, back during the whole Pokemania craze. And being that Pokemon has been long-running for about 20 years now, I don't expect it to be in everyone and their mom's face (put not intended lol) like it was back then. The freshness, as far as for the general public goes, is gone, and that's ok, that's how fads and trends tend to be (also, I don't intend it to ever get at those levels again but that's ok anyhow). That said, in regards to popularity as video games and as a brand compared to other IP's in similar fields, yes, Pokemon still is popular, even 6 generations and 721 Pokemon later (soon to be 7 generations).
Any why is that? Well, for one thing, the games still continue to bring dough in (and to be fair, the main series games are the bread and butter of the Pokemon franchise). Even the least successful Pokemon game financial-wise still outranks many other games from other IP's (of the games released around the same time; this is another one of those spots where the term relativity comes into play). Furthermore, it has been stated that the Pokemon IP ranks #2 in terms of successful video game IP's (number 1 being the Mario IP).
By the way, here's an article I found earlier today in regards to this topic. It's about 2 years old now but it's message still rings true today: denofgeek.com/us/games/pok-mon/239407/how-pokemon-survived-90s-
On the flip side of things, there's also this one (also an article about two years old): siliconera.com/2014/12/01/pokemons-audience-growing-older/
In regards to the anime: while older folks may not be watching it anymore, that's ok still as those older folks just get replaced by newer and younger kids coming into the fandom (and if that's fine with the show's creators, then that's cool I guess; like I mentioned, it's the games where the bread and butter are at). And let's be completely honest with ourselves, the anime is mostly geared towards kids. Some older fans simply grow out of it (an argument can be made for the games on this as well but for the sake of this paragraph, we'll stick with referencing the anime); say what you want about the anime's current state but let's face it, almost anything that seemed good when we were kids isn't as good as we thought it was when we get older (yes, we may have nostalgic feelings towards the earlier seasons but still, take off the rose-tinted nostalgic comments and you'll see what I mean; doesn't mean an older person can't try to find enjoyment out of watching it though). And again, it all comes back to the whole Pokemania craze and the fad wearing off (this is also evident by the fact that Pokemon movies don't come out in theaters anymore like the 1st 4-5 did). Furthermore, anime as a genre in and of itself hasn't really ever been a mainstream thing, at least in the US. A lot of it has mostly cult followings, catering to niche groups. And you know what? That's ok. When it comes to the anime, catering to the kids demographic has worked for The Pokemon Company (sure, it may feel repetitive and formulaic at times, especially every time that Ash goes to a new region, but even then, that's just an opportunity for new kids and new fans in general to jump in and if that works for the company, then that's fine). [To be honest, the topic of the anime is for another time and another as it's an interesting one in and of itself]
All in all, Pokemon, even the games themselves, cater to the youth of the time, and every generation introduced is just an opportunity for a new batch of youth to come into the fandom. Some fans may leave but as long new fans come in, it's fine. Furthermore, what makes the Pokemon fandom still going strong is its dedicated fanbase (like everyone who visits this site for example). While the rest of the world may not be into Pokemon like the late 1900's/early 2000's Pokemania had it appear, the numerous hardcore fans of the games have kept it going due to their devotion, and if Nintendo and GF are content with that, so be it.
Another thing too: Pokemon is still a hit in Japan and if it's still going strong in Japan (games, anime, etc) then I don't see its popularity and revenue going away, even if it may seem that way in the US (as other countries at the international level seem to be more embracing of Pokemon in their respective public spheres).
In regards to Pokemon Go, while yes, it may have seemed that Pokemon is back in the public sphere as far as the mainstream goes, don't let that fool you. There's already many people who have gotten bored of it (I even read a report where it stated that about 70% of players who started playing it last month are the ones that are still active; here's another report: attackofthefanboy.com/news/pokemon-go-lost-10-million-daily-users-beginning-august/ ) and unless Niantic starts adding new features to the app to keep things fresh and overall keep the momentum going, then the number of participants will keep dropping. Case in point; I read a tweet the other day from some guy stating that playing the game eventually started to feel like a chore for him, and I can kind of see what he means as a lot of time one ends up catching mostly Pidgeys and Rattata's lol. So yeah, if new things don't get introduced to the game, it's just going to end up in fad territory.